1. Field of the Invention
The invention relates to the field of acoustic detectors, and in particular, to leak detection equipment with active noise cancellation.
2. Related Art
When water or other fluids leak from underground pipes, quick and accurate determination of the site of the leak is necessary to reduce the amount of damage caused by leaking fluid. Acoustic sensing methods are used to locate leaks in underground pipes by detecting the vibrations caused by leaking fluids. Fluids leaking from underground pipes under pressure typically produce acoustic vibrations with a frequency in the range of about 40 Hz to about 4000 Hz.
In order to detect the acoustic vibrations, a transducer placed in contact with the ground converts the mechanical vibration into an electrical signal. The electrical signal is filtered to block most noise at frequencies below about 400 Hz and above about 2000 Hz. The signal can also be amplified before and/or after filtering. In some detectors, individual band-gap filters can be selected in order that ranges of frequencies can be monitored. The range of frequencies being monitored depends on the nature of the pipe, the material leaking from the pipe, the size of the leak, and the characteristics of the earth in which the pipe is buried.
The processed electrical signal can then be input to one or more speakers in a set of headphones, where it is converted back into acoustic vibrations. An operator wearing the headphones listens for the characteristic tone of the leak. The position of the transducer on the ground is varied in order to find the source of the leak. The operator must be able to accurately determine the spot at which the characteristic tone has a maximum volume in order that an accurate location of the leak is determined.
During leak detection, the sound reaching the operator's ear is primarily a combination of the sound attributable to the leak, noise picked up by the transducer and passed through the electronics to the headphones, and ambient noise transmitted through the air and through the headphone structure. An operator's ability to locate sound precisely depends in part on how well the sound of the leak can be distinguished over other sounds.
Ambient noise protection headphones, which are commonly utilized to reduce ambient background noise, have several disadvantages. First, the attenuation efficiency is limited by the quality of the seal to the operator's ears and by the characteristics of the foam cushions. Therefore, ambient noise protection headphones may not attenuate noise sufficiently to allow an operator to accurately determine the source of the leak. Second, the attenuation is indiscriminate. Sounds that the operator may need to hear, such as the sound of a co-worker's yelling a warning, are attenuated along with the unwanted background noise. Therefore, ambient noise protection headphones may pose a safety risk to the operator.
Active Noise Cancellation (ANC) headphones, which cancel unwanted noise instead of merely attenuating it, provide a better solution. ANC headphones contain microphones that convert environmental noise to an electrical signal that can then be utilized to produce sound of equal amplitude but opposite phase of the ambient noise. The signal from the leak detector, as with a CD or DVD player, can be input and the operator can monitor the sound produced by the leak detector while canceling ambient noise at the headphones.
ANC headphones are marketed by, for example, Bose Corporation of Framingham, MA, Sony Corporation of Tokyo, Japan, and Sennheiser Electronic Corporation of Old Lyme, CT. However, existing ANC headphones are not designed to operate effectively in the frequency spectrum of interest for detecting leaks, e.g. about 40 Hz to about 4000 Hz. For example, the Sony MDR-NC5 has active noise cancellation that operates to a maximum frequency of 1500 Hz, but has a 15 dB noise cancellation only at frequencies less than 300 Hz. The Sennheiser HDC451-1 has 10 dB noise reduction between 400 Hz, and has maximum operating frequency of 1000 Hz. Additionally, these noise cancellation headphones do not cancel noise detected by the acoustic detector of the leak detector. Also, providing noise cancellation for all noise in the spectrum may present a safety hazard to the operator, who then can not hear warning shouts or traffic noise.
Therefore, in order to increase operator safety and accuracy, an acoustic leak detection system with active noise cancellation is desired.